Solar collectors

ABSTRACT

The invention concerns a solar collector ( 10 ) which has at least one radiation-transmitting prism ( 16 ) which is wedge shaped in cross section. The prism has major side surfaces ( 20, 21 ) converging at an acute angle to a relatively narrow, operatively upper end ( 24 ). The opposite, lower end ( 26 ) of the prism is wider. A refractor ( 18 ) is arranged over the prism to refract solar radiation incident thereon onto the major side surfaces of the prism, as the sun moves relative to the earth, at angles allowing such radiation to enter the prism and be internally reflected therein towards a target at or adjacent the relatively wide end of the prism. The configuration allows high levels of solar concentration to be achieved.

BACKGROUND TO THE INVENTION

THIS invention relates to solar collectors.

There exist numerous devices designed to concentrate solar radiation forthe purpose of generating electricity or heat. In the case ofelectricity generation, the function of the solar collector is toconcentrate the radiation onto relatively small photovoltaic (PV) cells,while in the case of heat generation, the function of the collector isgenerally to concentrate the radiation onto a conduit or containerconveying or storing a fluid, such as a liquid or gas, the temperatureof which is to be elevated.

In the known devices it is recognised that for efficient collection andconcentration of the solar energy it is necessary for the device totrack the sun as the position of the sun relative to the earth changesduring the year and/or as the position of the sun relative to the earthchanges during the day. A single-axis system aligned N-S (north-south)should track the sun E-W (east-west) during the day, while a single axissystem aligned E-W should track the sun N-S during the year.

Concentrator systems that employ focusing lenses for primaryconcentration require either biaxial tracking, i.e. both N-S and E-W, ora secondary tracking system that varies the position of the lens ortarget in order to ensure that the collected radiation is focusedcorrectly on the target, i.e. PV cells or fluid conduit or container.The latter type of system, frequently referred to as a 1.5 timestracking system, typically moves the assembly of lenses, associatedreflectors and/or target either individually or in arrays. The apparatusrequired to achieve such movement can however be expensive andcomplicated.

Where electricity is to be generated with the use of PV cells an addeddisadvantage of systems which employ a focusing lens is the fact thatdirt particles on the lens create shadows which result in unevendistribution of radiation on the PV cells. Apart from the fact that thisreduces the efficiency of the PV cells, it can also cause permanentdamage to the cells. Dirt particles on the reflectors of areflector-type concentrating system can also be problematical.

One example of a known solar collector, described in U.S. Pat. No.4,282,862, uses an assembly of parallel wedges to reduce the angulardispersion of incident solar radiation. Radiation refracted by thewedges is then transported to the target by internal reflection in thinmodules composed of wedge-shaped glass elements. A disadvantage of thesystem is however a relatively low concentration ratio of around 2:1.“Concentration ratio” refers to the ratio of the area of the solaraperture, i.e. the area on which the solar radiation is incident, to thearea of the target onto which the radiation is concentrated. The lowconcentration ratio is indicative of a low level of efficiency. Anotherexample, described in U.S. Pat. No. 4,344,417, makes use of a narrow,wedge-shaped collector to receive incident radiation and reflect itinternally to the target area. The concentration ratio is however againrelatively low, indicating a low level of efficiency.

Further examples of prior art collectors are described in JP 11305130and JP 62266879. In the former case, the collector has wedge-shapedprisms and external reflectors arranged at a divergent angle withrespect to one another in order to collect radiation over a larger solaraperture and to concentrate such radiation, by both internal reflectionin the prisms and external reflection from the reflectors, onto a solarbattery. In the latter case N-S aligned, connected wedge-shaped prismsare again used to concentrate incident radiation by internal reflection.The prism assembly is used in conjunction with a conventional solarpanel.

It is an objective of the present invention to provide a novel andefficient solar collector.

SUMMARY OF THE INVENTION

According to the present invention there is provided a solar collectorcapable of single axis tracking and comprising:

at least one radiation-transmitting prism which is wedge shaped in crosssection and which has major side surfaces converging at an acute angleto a relatively narrow, operatively upper end of the prism, the prismhaving an opposite, operatively lower, relatively wide end; and

a refractor arranged over the prism to refract solar radiation incidentthereon onto the major side surfaces of the prism, as the sun movesrelative to the earth, at angles allowing such radiation to enter theprism and be internally reflected therein towards a target at oradjacent the relatively wide end of the prism.

There will typically be a plurality of the prisms assembled side by sidewith their narrow ends extending parallel to one another, and thecollector is preferably configured for single axis tracking in a planetransverse to the narrow ends of the prisms. In an arrangement in whichthe narrow ends of the prisms extend N-S in use, the collector ismovable to track the sun in an E-W plane during the course of a day,typically with means for rotating the collector about a N-S axis.Alternatively, in an arrangement in which the narrow ends of the prismsextend E-W in use, the collector is movable to track the sun in a N-Splane during the course of a year, typically with means for rotating thecollector about an E-W axis.

The preferred refractor is a linear refractor, in particular a linearFresnel lens.

The narrow ends of the prisms may be adjacent to or in contact with therefractor, or they may be spaced from the refractor. In the latter casethe collector may include a reflector arrangement configured to reflectradiation incident thereon at angles appropriate for acceptance thereofby the prism for internal reflection therein, such as an arrangementincluding convergent reflectors which stand up from the refractor overthe narrow ends of the prisms and are arranged to reflect solarradiation outwardly onto the refractor.

The collector may be located beneath a radiation transmitting cover, forinstance in greenhouse or building heating application.

For improved concentration of the solar radiation, the collector mayinclude a radiation transmitting secondary solar concentrator at thewider end of each prism. This may have side walls, typically planar orconcave, which converge towards one another to a width less than that ofthe wider end of the prism. The secondary solar collector should be madeof a material with a higher refractive index than the material of whichthe prism is made. Also, the prism and secondary solar concentratorshould meet one another at a curved, typically an upwardly convex,interface.

The various embodiments of the invention described below can be used inan electricity generating mode in which case there will be a PV cell atthe wider end of the prism or at the end of the secondary solarcollector, or in a fluid heating mode in which case there will be a pipeconveying a fluid which is to be heated at the wider end of the prism orat the end of the secondary solar concentrator.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in more detail, by way of exampleonly, with reference to the accompanying drawings.

In the drawings:

FIG. 1 shows a diagrammatic plan view of a solar collector according toone embodiment of the invention;

FIG. 2 shows a diagrammatic cross-section at the line 2-2 in FIG. 1;

FIG. 3 shows an enlargement of the circled area in FIG. 2;

FIG. 4 shows a cross-section at the line 4-4 in FIG. 1;

FIG. 5 shows a diagrammatic plan view of a solar collector according toa second embodiment of the invention;

FIG. 6 shows a diagrammatic cross-section at the line 6-6 in FIG. 5;

FIG. 7 shows a cross-section at the line 7-7 in FIG. 5;

FIG. 8 shows a diagrammatic plan view of a solar collector according toa third embodiment of the invention;

FIG. 9 shows a diagrammatic cross-section at the line 9-9 in FIG. 8;

FIG. 10 shows an enlargement of the cross-sectional view seen in FIG. 9;

FIG. 11 shows a cross-section at the line 11-11 in FIG. 8;

FIG. 12 shows a diagrammatic plan view of a solar collector according toa fourth embodiment of the invention;

FIG. 13 shows a diagrammatic cross-section at the line 13-13 in FIG. 12;

FIG. 14 shows a cross-section at the line 14-14 in FIG. 12;

FIG. 15 shows an enlargement of the cross-sectional view seen in FIG.13; and

FIG. 16 illustrates a secondary solar concentrator which can be used inthe embodiments illustrated in the earlier Figures.

In the Figures, the letters N, S, E and W refer respectively to north,south, east and west.

DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

FIGS. 1 to 4 illustrate a first embodiment of solar collector accordingto this invention. It includes a module 10 having a rectangular boundingframe 12 which supports at an assembly 14 of side-by-side, parallelgenerally wedge-shaped prisms 16 of, for instance, glass, acrylic orpolystyrene as well as a linear refractor 18, typically in the form of alinear Fresnel lens. The linear refractor 18, which may also be ofglass, acrylic or polystyrene, is in use exposed to solar radiation andmay include an ultraviolet (UV) filter.

The individual prisms 16 are elongate both in a vertical sense and ahorizontal sense. Referring in particular to FIG. 3, each prism hasmajor, planar side surfaces 20 and 21 which converge at an acute angle22, in this case 3°, to one another towards a relatively narrow end 24of the prism. The opposite end 26 of the prism is relatively wide andhas mounted to it a series of PV cells 28 arranged side by side with oneanother in a direction into the plane of the paper in FIG. 3. The cellsare in turn mounted in contact with aluminium heat sinks 30 which removeexcess heat from the cells. The numeral 32 indicates an axis about whichthe module 10 can be rotated.

In this embodiment, the narrow ends 24 of the prisms are attached to theunderside of the linear refractor. Although the narrow ends are shown assharp edges, they may in practice be slightly truncated.

In FIG. 1, the prisms are arranged operationally with their narrow edges24 extending N-S, and the linear refractor is designed to refract solarradiation incident thereon onto the major side surfaces 20 and 21 of theprisms.

The numerals 34 in FIG. 3 indicate solar rays, assumed to be parallelwhen incident upon the linear refractor. During the course of each year,the sun moves relative to the earth, and the module 10, in a N-Sdirection. With the sun at equinox solar rays 34.1 incident upon thelinear refractor are refracted towards the lower, i.e. wider end 26 ofthe illustrated prism 16. The angle at which the rays fall upon themajor surface 20 of the prism 16 is within the acceptance angle of theprism, i.e. the rays enter the prism and are not externally reflected atthe prism/air interface. As the sun moves towards the solstice positionsduring the course of the year the rays impinge at successively higherpositions on the surface 20 as exemplified by the numeral 34.2. In eachcase, rays which enter the prisms are totally internally reflected andare eventually incident upon the PV cells 28. To increase the acceptanceangle of the prism, the surfaces 20 and 21 may be externally coated witha non-reflective coating.

For a prism having the given dimensions and a refractive index of 1.4,solar rays refracted at an angle 36 will be within the acceptance angleof the prism and are accordingly received by the prism and internallyreflected therein to be incident on the PV cells. Thus all rays 34incident on the linear refractor over the given lateral dimension 38 of72 mm, corresponding to the solar aperture of the module 10, will reachthe PV cells. These cells have a lateral dimension of the order of 5 mmfor the given prism dimensions. Thus the module 10 provides aconcentration ratio of 72 mm:5 mm, i.e. approximately 14:1.

It is perceived that with this relatively high concentration ratio, themodule 10 will be able to function very efficiently as a solarcollector.

During the course of the day, the sun moves relative to the earth froman easterly to a westerly position. The module 10 is rotated at theappropriate angular speed about the axis 32 in order to track the sunduring this relative movement. Thus the collector illustrated in FIGS. 1to 4 requires single axis tracking only to take account of differentsolar angles during the course of the day. It is believed that it willbe possible to achieve such single axis tracking in a simple, reliableand economical manner without the necessity for complicated arrangementsto vary focal length as in 1.5 times tracking systems.

It will also be understood that for each latitude position of the sun,the solar rays are refracted parallel to one another on each side of theprism by the linear refractor 18. Thus all solar rays incident on thelinear refractor 18 within the illustrated solar aperture, i.e. withinthe lateral dimension 38, will be concentrated onto the PV cells 28. Theside by side spacing of the prisms 16 will accordingly be selected toensure that all radiation incident on the refractor 18 is captured andconcentrated.

FIGS. 5 to 7 illustrate a second embodiment of the invention. In theseFigures, components corresponding to those in FIGS. 1 to 4 aredesignated with the same reference numerals.

A major difference between the second embodiment and the firstembodiment is the fact that the narrow end 24 of each prism is spacedvertically below the linear refractor 18. In this case, the refractor 18is formed with a gap 40 aligned with the central, vertical axis of theprism 16, and the gap is spanned by a reflector structure 42 composed ofupstanding reflector panels 44 arranged at an acute angle to oneanother. In this case solar rays 34.7 which would not be refracted bythe refractor 18 at an angle acceptable to the prism, i.e. the rayswould otherwise be externally reflected by the surfaces 20 and 21 of theprism, are reflected by the reflector panels to angles which result inacceptance by the prism. In this way it is possible to increase thesolar aperture 38 to a dimension of 144 mm for the other, givendimensions. In this case, a concentration ratio in excess of 20:1,corresponding to high efficiency of the solar collector, can be obtainedwith relatively small additional cost attributable to the provision ofthe reflector structure 42.

In the embodiment of FIGS. 5 to 7 the prisms are, as in FIG. 1, alignedN-S and the facility is again provided for rotation about the N-S axis32 in order to track the sun during the course of the day.

FIGS. 8 to 11 illustrate a third embodiment. Once again, like componentsare designated by like numerals. In this case, the prisms 16 are alignedE-W and the facility is provided for N-S tracking.

The linear refractors 18 are arranged in a curved shape as showndiagrammatically In FIG. 9, within a light-transmitting greenhouse dome58. Referring to FIG. 10, each prism is, as in the second embodiment,spaced some distance below a refractor 18. In this case, the target foreach prism is a PV solar cell 27 mounted on a fluid pipe 28 throughwhich a fluid such as water is conveyed. The wider end of the prism isconnected to the pipe 28. The prism is connected to the refractor 18 bylight transmitting side panels 50, possibly of acrylic, which alsoprovide structural integrity.

The pipe 28 is rotatable about its own E-W aligned axis in order totrack the sun during the course of the year. Rotation of the pipe isaccompanied by rotation of the prism 16 and refractor 18. One or morecounterweights (not shown) may also be provided to assist the rotationalmovement. The numerals 34.7 indicate solar rays refracted by therefractor 18 at mid-day for different latitude angles of the sun whilethe numerals 34.8 indicate solar rays refracted by the refractor attimes early and late in the day, for example 08h00 and 16h00, again fordifferent latitude angles of the sun. In the mid-day position, the raysare refracted to the wider end 26 of the prism while in the earlymorning and late afternoon positions, the rays are refracted to thenarrow end of the prism. In each case, internal reflection within theprism transports the radiation to the target area. These solar positionsare also indicated in FIG. 11.

With the dimensions given in FIGS. 8 to 11, concentration ratios of theorder of 28:1 can be obtained.

In another embodiment similar to that of FIGS. 8 to 11, pipe(s) 28 couldbe arranged to move in an arc, in a N-S plane as in FIG. 9, as opposedto rotating.

The embodiments of FIGS. 1 to 4 and 5 to 7 are described above inelectricity generating applications. It will however be understood thatsuch embodiments could also be used for water or other fluid heatingduties, in which case the PV cells would be replaced by fluid transferpipes, fluid containers or the like. It will also be understood that inthis application there would be a requirement for fluid pipe connectionsable to take account of rotational movements about the axis 32. Theembodiment of FIGS. 8 to 11 is considered particularly appropriate forfluid heating, particularly in situations where the pipe 28 rotatesabout its axis and accordingly does not change position.

In other embodiments of the invention, not illustrated, PV cells couldbe embedded during moulding in the wider ends of the prisms 16.

FIG. 16 illustrates a modification in which a secondary solar radiationconcentrator is indicated by the reference numeral 70. The concentrator70, which extends for the full length of the prism 16, is a solid orliquid body made of a material having a higher refractive index than thematerial of which the prism is made. In one example, the prism is madeof an acrylic, such as PMMA (Polymethyl methacrylate) having arefractive index of less than 1.5 and the secondary concentrator 70 ofpolystyrol or glass having a refractive index of more than 1.5.

The secondary concentrator is placed at the wider, lower end of theprism 16 and is intimately connected to the prism at an upwardly convexinterface defined by a convex surface 72 of the secondary concentratorand a concave surface 74 of the prism. The secondary concentrator 70 hasplanar side surfaces 76 and 78 and a planar lower surface 80 to which,in this example, a heat transmitting coupler 81 is intimately attached.The coupler 81 is in intimate contact with the pipe 28.

The numeral 82 indicates a solar ray which enters the prism 16 throughthe side surface 20, is refracted at the prism/air interface and travelsthrough the lower part of the prism to the convex interface between theprism and the secondary concentrator 70. At this interface the ray isrefracted into the secondary concentrator and is thereafter reflectedinternally for eventual impingement on the coupler 81. It will beunderstood that other solar rays that have been internally reflected inthe prism will likewise be refracted into the secondary concentrator 70for subsequent passage directly or through internal reflection onto thecoupler.

To ensure that rays which enter the secondary concentrator 70 arereflected onto the coupler 81, inwardly facing mirrors 84 (only oneshown) may be placed against the surfaces 76 and 78 or these surfacesmay themselves be mirrored.

Instead of side surfaces 76 and 78 which are planar, the side surfacesof the secondary concentrator may be concave as indicateddiagrammatically by the numeral 86, or convex.

The convex interface defined by the surfaces 72 and 74 is preferred to aplanar, horizontal interface because it will tend to refract radiationin the appropriate direction for subsequent reflection onto the coupler81. A secondary concentrator having a convex interface as illustratedmay be referred to as a secondary convex concentrator (SCC).

As exemplified above it is preferred that the refractive index of theSCC be greater than that of the prism 16 in order to ensure that solarrays are appropriately refracted.

It will be understood that the SCC seen in FIG. 16 will increase theconcentration ratio further, implying high levels of solar concentrationefficiency. It will furthermore be understood that the SCC could equallywell be used to achieve highly efficient concentration of solarradiation onto a PV cell in place of the coupler 81 and pipe 28 inelectricity generating applications.

SCCs such as that described above can be used in conjunction with theprisms 16 in any of the embodiments seen in the drawings.

FIGS. 12 to 15 illustrate another embodiment of the invention in whichSCCs 70 are used. In this embodiment, the numeral 60 indicates anenclosure mounted for example in a fixed position on a building (notshown). The enclosure 60 has a light-transmitting roof panel 62,possibly made of glass or a fluoropolymer. Solar collection units 63,each including a linear refractor 18, prism 16 and SCC 70, are supportedby bearings 64 fixed in spaced metal frames 66. In each case a heat pipeis rotatable in the associated bearing and the linear refractor 18 issupported by support arms or radiation-transmitting sheets 67. Each unitincludes a counterweight 68 connected to the associated heat pipe by anarm 69. The counterweight may, for instance, be provided by a weight orby a length of heavy rod or pipe extending parallel to the associatedprism 16.

Like the embodiment of FIGS. 8 to 11, the embodiment of FIGS. 12 to 15employs single axis tracking, for each unit about an E-W axis, to trackthe sun as it moves relative to the earth during the course of the year.Although the units 63 are shown as independent of one another it will beunderstood that they would in practice be linked and would movesynchronously.

As in FIG. 10 the numeral 34.7 in FIG. 15 designates solar raysrefracted by a refractor 18 at mid-day for different latitude angles ofthe sun while the numeral 34.8 designates solar rays refracted by therefractor at times early and late in the day, for example 08h00 and16h00, again for different latitude angles of the sun.

Apart from the simplicity of the single axis tracking systems which areemployed in the embodiments described above, and the high concentrationratios and favourable efficiency which can be obtained with suchembodiments, an important advantage of these embodiments, compared toknown system using focusing lenses, is the fact that internal reflectionby the prisms ensures that radiation is evenly distributed across thereceiving surfaces of the PV cells in electricity generatingapplications, and that shadows attributable to dirt particles on thelenses do not occur.

1. A solar collector comprising: at least one radiation-transmittingprisms which is wedge-shaped in cross-section and which has major sidesurfaces converging at an acute angle to a relatively narrow end of theprism, having an opposite, relatively wide end; and a refractor arrangedover the prism to refract solar radiation incident thereon onto themajor side surfaces of the prism, as the sun moves relative to theearth, at angles allowing such radiation to enter the prism and beinternally reflected therein towards a target at or adjacent therelatively wide end of the prism.
 2. A solar collector according toclaim 1 comprising a plurality of the prisms assembled side by side withtheir narrow ends extending parallel to one another.
 3. A solarcollector according to claim 2 wherein the collector is configured forsingle axis tracking in a plane transverse to the narrow ends of theprisms.
 4. A solar collector according to claim 3 wherein the narrowends of the prisms extend N-S in use and the collector is movable totrack the sun in an E-W plane during the course of a day.
 5. A collectoraccording to claim 4 comprising means for rotating the collector about aN-S axis.
 6. A solar collector according to claim 3 wherein the narrowends of the prisms extend E-W in use and the collector is movable totrack the sun in a N-S plane during the course of a year.
 7. A solarcollector according to claim 6 comprising means for rotating thecollector about an E-W axis.
 8. A solar collector according to claim 1wherein the refractor is a linear refractor.
 9. A solar collectoraccording to claim 8 wherein the refractor is a linear Fresnel lens. 10.A solar collector according to claim 1 wherein the narrow ends of theprisms are adjacent to or in contact with the refractor.
 11. A solarcollection according to claim 1 wherein the narrow ends of the prism'sare spaced from the linear refractor.
 12. A solar collector according toclaim 11 wherein the collector includes a reflector arrangement toreflect radiation incident thereon at angles appropriate for acceptancethereof by the prism for internal reflection therein.
 13. A solarcollector according to claim 12 wherein the reflector arrangementcomprises convergent reflectors which stand up from the refractor overthe narrow ends of the prisms and are arranged to reflect solarradiation outwardly onto the refractor.
 14. A solar collector accordingto claim 2 wherein the collector is located beneath a radiationtransmitting cover.
 15. A solar collector according to claim 2 whereinthe prisms are spaced from the refractor and are connected to therefractor by radiation-transmitting side panels.
 16. A solar collectoraccording to claim 1 comprising a radiation transmitting secondary solarconcentrator at the wider end of each prism.
 17. A solar collectoraccording to claim 16 wherein the secondary solar concentrator has sidewalls which converge towards one another to a width less than that ofthe wider end of the prism.
 18. A solar collector according to claim 17wherein the side walls are planar or concave.
 19. A solar collectoraccording to claim 16 wherein the secondary solar collector is made of amaterial with a higher refractive index than the material of which theprism is made.
 20. A solar collector according to claim 17 wherein theprism and secondary solar concentrator meet one another at a curvedinterface.
 21. A solar collector according to claim 20 wherein a convexsurface of the secondary solar concentrator mates with a concave surfaceof the prism at the interface.
 22. A solar collector according to claim1 comprising a PV cell at the wider end of each prism.
 23. A solarcollector according to claim 16 comprising a PV cell at an end of thesecondary solar collector remote from the prism.
 24. A solar collectoraccording to claim 1 comprising a pipe conveying a fluid which is to beheated at the wider end of the prism.
 25. A solar collector according toclaim 16 comprising a pipe conveying a fluid which is to be heated at anend of the secondary solar concentrator remote from the prism.
 26. Asolar collector according to claim 1 wherein each prism is made ofglass, acrylic or polystyrene.
 27. A solar collector according to claim16 comprising a pipe conveying a fluid which is to be heated at an endof the secondary solar concentrator remote from the prism.